1. Field of the Invention
This invention relates to a process for slurry mining coal and a process for treating a coal bed, which has been made permeable, with a heat-carrying fluid to recover energy values from an underground coal formation.
2. Prior Art
It is generally known that coal is removed from the ground using two methods, either strip mining, in which the coal is merely dug out of the ground by mechanical or hydraulic means and transferred to the place of use, or underground mining using methods such as slurry mining (see U.S. Pat. No. 3,260,548 to Reichl), room and pillar, or longwall. The means for taking the coal out of the ground in the room and pillar or longwall methods are generally mechanical cutters, rippers, planers, loaders, etc. In slurry mining hydraulic apparatus is used to direct pressurized water at the coal seam to disaggregate the coal and form a slurry which is then pumped out of the mine to the surface. In all of these coal mining techniques it is first necessary to loosen the coal from the formation using some means such as explosives, hydraulic pressure, or physically contacting the coal with cutters, etc., before the coal can be transported away from the mine and to the place of use. Such methods require much time and large capital outlays for expensive loosening equipment.
Attempts have been made in the coal industry to find an agent which would attack the coal in such a way that the bonds between the coal constituents would be weakened and mechanical separation of the coal could be facilitated. Such a process is taught by U.S. Pat. No. 1,532,826 to Lessing, wherein the coal is treated with an acid or an aryl amine to facilitate mechanical segregation of the coal. Although such an acid treatment facilitates disaggregation by mechanical means the treatment does not result in complete disaggregation of the coal as does an acid treatment in phosphate mining as described in U.S. Pat. No. 3,359,037 to Every and Hughes.
In the past, attempts have been made to treat coal by various methods in order to recover, either in situ or after the coal had been removed from the mine, liquid or gaseous fuels therefrom. Because of the costs involved in first removing the coal from the ground it would be preferred to carry out the treatment for recovery of fuels from the coals in situ. Generally such in situ attempts have not been very successful because, i.a., the coal seam is generally impermeable to the recovery fluids injected into the formation and thus the coal present in the formation cannot be sufficiently contacted by the fluid to obtain an adequate recovery. U.S. Pat. No. 2,595,979 to Pevere and Arnold teaches a method of liquification of coal wherein the underground coal is first contacted with hydrogen at about 550.degree. F. or more and 1000 pounds per square inch pressure. The residue remaining can then be treated with an oxygen containing gas to gasify the residue. However, the process is slow because the gases do not contact much of the coal surface area at any one time.
It has now been discovered that by contacting a coal formation with a basic aqueous solution, particularly sodium hydroxide, the coal can be substantially disintegrated. Although it has been generally known that the finely ground bituminous coal can be treated with an aqueous alkali solution at elevated temperatures to obtain a coke-like residue, and that the hydrogenation of these residues forms products which are more hydrocarbon-like in nature than does a similar hydrogenation of the coal itself, (see for example "Action of Aqueous Alkali on a Bituminous Coal" by Leo Kasehagen in Industrial and Engineering Chemistry, May, 1937), it was surprising indeed to discover that the coal substantially disintegrates when treated with a basic aqueous solution. This phenomena can be utilized to disintegrate a coal formation so that coal can be mechanically removed more easily or removed by slurry mining. Further, if sufficient porosity is maintained in an underground cavity to permit subsequent fluid injection, reactions by these fluids can be used to recover further chemical or energy values from the coal.